1. Field of the Invention
This invention relates generally to the administration of local anesthetic and more particularly to so-called epidural catheters for the injection of local anesthetic in the epidural space in the spine.
2. Preliminary Discussion
The name of a popular epidural catheter, Flextip™, itself infers that the distal end is very flexible and more proximal regions of the catheter are less so. Inasmuch as styleted epidural catheters are, for normal uses, professionally unacceptable and have been so for the better part of fifteen years, any viable soft tip epidural must solve the problem of stiffening that section of catheter which must be pushed to advance the catheter past the curved tip of the introducing epidural needle and into the patient's epidural space. It will be apparent to those skilled in the art that this stiffened section must of necessity be located within the epidural needle's length from the distal end of the catheter being inserted. Were this not so, in use when resistance is encountered as the catheter's distal tip transits the tip of the needle and attempts to enter the epidural space, the anesthesiologist would be unable to overcome this resistance by pushing on the extremely flexible and essentially ‘unpushable’ soft distal tip of the catheter which has not yet entered the needle hub. It will be equally apparent to those skilled in the art that the length of the stiffened section can effectively control the maximum depth to which the catheter can be inserted, in this manner minimizing the likelihood of the catheter's soft tip curling back upon itself and creating a knot that seriously complicates later removal.
3. Discussion of the Prior Art
The first Flextip, disclosed in U.S. Pat. No. 5,004,456 to Botterbusch and Frankhouser, provided for a stiffer proximal section of the catheter with a solid, higher durometer polyurethane tube butt-welded to a ‘less than needle's length’ section of softer polyurethane tubing.
The Flextip Plus and its later mimics achieve proximal stiffness with a unifilar stainless steel wire coil. The initial tension of the coil provides the catheter with column strength and pushability up to a buckling load, judged initially by some to be less than desirable, especially in fine gage (smaller diameter) catheters. A ‘less than needle's length’ of highly flexible, stretched coil at the distal end assures a soft, atraumatic tip of a length insufficient to interfere with insertion into the needle.
The catheter described in U.S. Pat. No. 5,947,940 to Beisel offers high strength and low marginal cost, being based on a continuously wound, coated, and processed structure. Despite offering some disclosure about ‘providing regional stiffness,’ an acceptable method of providing this required regional stiffness has remained illusive.
Vitullo et. al. in U.S. Pat. No. 6,641,563 and U.S. patent application Ser. No. 0040030289 allude to the unsolved problem, that is the need to employ a stylet for inserting such catheters, and deem them less than totally satisfactory.
If one cannot provide the soft tipped catheters with adequate regional stiffness required for insertion into the needle, an attempt to improve thread assist devices, or TADs, seems reasonable. A simple TAD, in the form of a male luer adapter with a small hole that effectively straightens the epidural catheter in its passage through the epidural needle hub, is the very device that allowed first development of stylet free epidurals. Though relatively stiff in comparison to newer flextips, the slightly softer simple tube catheters could be inserted without a stylet using this simple TAD to eliminate the depth of the female luer hub from the column being pushed and, by so shortening the unsupported length, increasing its column strength to a level adequate for insertion.
With the later advent of helical reinforcement in catheters, epidural catheters' distal tips could become much softer, thereby all but eliminating venous cannulation and paresthesiae during their insertion; these simple TADs were, however, unusable with catheters based on U.S. Pat. No. 5,947,940.
Vitullo, et. al. teaches in U.S. Pat. No. 6,641,563 and U.S. patent application Ser. No. 0040030289 the use of an externally applied UV curable or heat-shrinkable sleeve to achieve regional stiffening. Although UV curable materials can be die-coated in continuous deposits, intermittent deposits are very difficult to achieve at best. In addition, UV cured materials lack the optimal tensile properties to enable them to serve well in reasonable thicknesses. Both of the above-mentioned external stiffeners present the distinct disadvantage of locally increasing the catheter's O.D. and thereby limiting useable, already precious wall thickness through the length of the catheter. Inasmuch as the epidural needle determines the largest diameter that will pass, the diminished wall thickness available imposes further limitation on the combined thickness of external tubular covering(s) and the thickness of any helical reinforcements. Decreasing these only slightly greatly diminishes the maximum internal pressure, overall strength and kink resistance of the catheter.
Samson teaches methods of catheter stiffening in several U.S. patents pertaining to interventional radiological catheters. These neurological and peripheral vascular devices, which are far more procedure specific and intricate than epidural catheters, are less burdened with cost constraints and therefore need not be made by continuous methods. In U.S. Pat. No. 6,258,080 and others previous Samson teaches the use of spiral, coaxial ribbon stiffeners, spiraling in one or both spiral directions, either uni- or multifilar, and of various widths wound upon an inner tubular liner before applying the outer coating. In U.S. Pat. No. 6,090,099 a metallic braid lies between at least one internal stiffener member and the exterior tubing member. Although the inner stiffening tubing is of length and wall thickness similar to those in the present invention, they are made by a method of construction which is entirely distinct, being built individually from within upon a mandrel, the outer cover being applied last by heat shrinking, utilizing mostly radiation crosslinked polyolefins. Furthermore, the melting point of the inner stiffening tubing is about equal to the shrink temperature of the exterior tubular member. The costs of these methods of construction are also simply too great to be useful in the production of epidural catheters.
Truckai, et. al., in U.S. Patent teaches a deflectable electrophysiology catheter with a flexible stiffener member sliceable within an axial lumen to be used to control catheter stiffness.
Racz, in U.S. Pat. No. 5,899,891 teaches a catheter structure utilizing a coil or other flexible means for increasing a tube's end strength adhered within either the proximal or distal end acting as an anchor for an axial cord laterally mobile within the catheter. These end modifications can ‘possibly (increase) its rigidity, but do not generally involve decreasing the tip's flexibility.’ They provide generally a method to reinforce the proximal end so as to anchor the intraluminal cord, on which the catheter's tensile integrity depends.
Carter, in U.S. Pat. No. 5,599,326 teaches a structure for vascular catheters that achieves stiffness control with an interior stiffener comprising a spirally cut tubing member, where the spiral pitch may be varied, and a gap introduced between slit sections to control stiffness. Similar to the case for Samson, v.s., these catheters are constructed individually on mandrels from the inside outward.
Larson teaches, in U.S. Pat. No. 6,475,209 a catheter with a spiral cut transition member disposed within the annular lumen between inner and outer tubes.
Jansen, et al., teach, in U.S. Pat. No. 6,638,316 the use of spiral wound stiffeners, the first metallic, the second non-metallic which result in a relatively stiff proximal segment and a relatively flexible distal segment into which only the second stiffener continues.
Chow, in various U.S. Patents including U.S. Pat. Nos. 5,976,120, 6,171,296 and 6,296,631 teaches construction of a catheter with changing flexibility by using reinforcing strand(s) which changes diameter from proximal to distal end of the catheter. These variations in reinforcing strands are not economical to employ in the manufacture of epidural catheters.
Le, et al., teach in U.S. Pat. No. 6,355,027, the construction of microcatheters by the application of resins of different Shore hardness, along and about the braid which overlies an inner resin layer, thereby establishing two regions of different stiffness.
There is still, therefore, an unmet need to make a strong, economical epidural catheter based on a continuously wound and extrusion-coated structure with regional stiffening adequate to allow insertion.